Terry Cavanagh's gravity abusing platformer VVVVVV is now available on Steam for £3.59/$4.49. I gave it 86% in PCG UK 211. If you have any love of platforming puzzlers at all, you'd be a fool to pass it up for pocket change like this.
In VVVVVV ( pronunciation guide ), you're the captain of a ship that's stranded inside a spacial anomaly, forced to leave the Comfy Chair and venture into the area's crazed physics to find your crew. It's a difficult platformer where you can flip gravity while you're touching the floor, but you can't jump. So if you see a spot you want to get to, you have to plot a zig-zag path between the floor and the ceiling.
It's a nonlinear game without powerups, unlocks, or any kind of weird barriers to entry. You can access any part of the game just by going there. It's really hard, and the music is really good, and you can buy it here .